In liquid crystal displays, a backlight source is used to illuminate through a liquid crystal panel from behind. In recent years, upsizing, thinning, weight reduction, longer life, or the like of liquid crystal displays, as well as the effort to improve video quality using blink control, have brought increasing attention to a light emitting device including a plurality of Light Emitting Diodes (LEDs) arranged on a substrate for surface emission. In such a light emitting device, the following two methods are mainly used to produce white light.
The first method to produce white light is to arrange LEDs each having R (red), G (green), and B (blue) three colors and to produce white light by synthesizing light beams in the three colors by lighting the LEDs simultaneously. The second method is to coat, for example, a blue LED with a phosphor-containing resin to convert blue color into white color. The structure including the blue LED coated with the phosphor-containing resin is called “white LED”.
Here, the first method is expensive because of the need to use LEDs in the three colors of R, G, and B. The second method also has a difficulty in forming the phosphor-containing resin regularly and evenly, because the phosphor-containing resin is potted in a small area of the LED.
Accordingly, as a substitute for the second method, the third method of converting colors by using a blue LED, together with a phosphor-containing resin sandwiched between sheet substrates or with a phosphor-containing sheet made of a phosphor-containing resin in the form of a sheet, is gaining attention. (Refer to Patent Literatures 1 and 2, for example.)
Some phosphors are susceptible to oxygen and moisture vapor. For example, sulfide phosphors, such as SrGa2S4:Eu, CaS:Eu, and SrS:Eu, have a sharp light emission spectrum and accordingly, are excellent phosphor materials that allow reproduction of a broad range of colors, and (Ba,Sr)3SiO5:Eu is a high brightness orange light emitting phosphor material. However, these phosphor materials are prone to deterioration under a high temperature and high humidity environment due to moisture vapor.
When these phosphors are in use, some sort of measures need to be taken to block moisture vapor. Although these phosphors can be hardly adopted for white LEDs mentioned in the above second method, measures may be still taken for the third method by, for example, covering a phosphor layer with a moisture vapor barrier film. For example, there have been proposed a method of providing, on a phosphor-containing resin, a protective layer of a silicon compound or the like (refer to Patent Literature 3) and a method of forming a moisture vapor barrier layer on a surface of a phosphor-containing resin (refer to Patent Literatures 4 and 5).